Bonaire National Marine Park - LAC - FLORA

On the southeast coast of Bonaire you will find the Dutch Antilles’ greatest lagoon: Lac.

Lac is described as an approximately 3-kilometer wide basin, also called a lagoon, which is positioned on the windward side of Bonaire.

Lac supports Bonaire's only significant mangrove forest and seagrass ecosystems. Seagrass beds cover the open water area of the bay. The landward edge is surrounded by an actively growing mangrove forest, which is systematically encroaching on the bay. The mangroves are particularly important as nesting and roosting areas for birds and the seagrass beds form nursery grounds for some important reef fish as well as important foraging areas for sea turtles. An extensive coral dam on the far east side creates a boundary between the lagoon and the open sea.

If you drive around Lac you will find similar flora to that of Washington Slagbaai National Park. Different types of cacti (Candle cacti(Pilosocereus lanuginosus, Subpilocereus repandus and Rittocereus griseus)) and other plants and trees, such as the Divi-divi (Caesalpinia coriaria), the Cossi (Acacia tortuosa) and Aloe (Aloe barbandensis), which are adapted to the dry and harsh environment, line the outer fringes of the mangrove system of Lac. The density of individual plants and different species is lower in comparison with Washington Slagbaai National Park, caused by the higher dynamic circumstances and the strong Eastern Trade winds.

The mangrove wetlands, seagrass meadows and coral reefs of Lac itself, show the visitor a totally different side of Bonaire’s flora. These three completely different areas harbour a high biological diversity and, together with the land nutrient run-off, form the basis of the coastal marine productivity. The primary productivity of mangrove, seagrass and coral reef ecosystems is assumed to be the basis of present and potential future yields of shallow water marine resources in the region. It has become increasingly apparent that Caribbean coastal ecosystems are degrading because of increasing anthropogenic stresses, which are superimposed upon natural local, regional and global trends. All this corresponds with the overall goal of Lac and the Bonaire National Marine Park.

“To protect the natural environment of Lac, along with the naturally occurring species, from degradation; and to preserve the aesthetic appeal of Lac as an unspoiled and undeveloped area while promoting recreational day use”

 

The mangrove forest:
Lac itself contains two different species of mangroves: the red mangrove (Rhizophora mangle) and the black mangrove (Avicennia germinans). These two species of mangroves grow both in salt- or brackish environments, but need fresh water to survive. The trees convert salt water into fresh water with the help of a semi-permeable membrane. This process is called reverse osmosis (intermezzo Biology of mangroves).

The mangrove forest of Lac is approximately 850 years old but will disappear in another 800 years. Bonaire is rising out of the sea at a rate of 0,03 mm per year. The membrane needs 60 cm of water pressure to function. So because Bonaire is rising, water pressure is decreasing and the membrane will no longer be able to supply fresh water and the soluble nutrients to the mangrove.

Another possible reason for the mangrove dieback is the shortage of fresh water inflow since the 1960s. In the 1960s a road (Rooi Grandi) was constructed to make Cai accessible for tourists and local people, by the use of gravel and other rocky material. The ‘heavy’ traffic using the road changed the porous road into an almost impermeable dam.

 Intermezzo: Biology of mangroves

In order to sustain a successful dominance in such a unique environment a plant must have evolved with some unique adaptations. This is true for both mangrove species in Lac: the red mangrove (Rhizophora mangle) and the black mangrove (Avicennia germinans).

High salinity concentrations:
The first major obstacle mangrove species have adapted to overcome is that of high salinity concentrations. In this case the black mangrove utilizes salt excreting glands, formed only under saline conditions. Red Mangroves exclude salt by having rather impermeable roots which are highly suberised, acting as an ultra-filtration mechanism to exclude Na salts from the rest of the plant. Any salt which does accumulate in the shoot is concentrated in old leaves which are then shed, as well as stored away safely in cell vacuoles.

Respiration:
Red mangroves, which can live in the most inundated areas, prop themseles up above the water level with stilt roots, and can then take in air through slits in their bark (lenticils). Black mangroves live on higher ground, and make many pneumatophores (specialised root-like structures which stick up out of the soil like straws for breathing) which are covered in lenticils.

Water loss:
Because of the limited availability of freshwater in the salty soils of the intertidal zone, mangrove plants have developed ways of limiting the amount of water that they lose through their leaves. They can restrict the opening of their stomata (small pores on their leaf surfaces which exchange carbon dioxide gas and water vapour during photosynthesis) and also have the ability to vary the orientation of their leaves. By orientating their leaves to avoid the harsh midday sun, mangrove plants can reduce evaporation from their leaf surfaces.
During the uptake of water the trees convert salt water into fresh water with the help of a semi-permeable membrane. This process is called reverse osmosis

Nutrient uptake:
The biggest problem that mangroves face is nutrient uptake. Because the soil that mangroves live in is perpetually water logged, there is not much free oxygen available. At these low oxygen levels, anaerobic bacteria proceed to liberate nitrogen gas, soluble iron, inorganic phosphates, sulfides, and methane, which help contribute to a mangrove's particularly pungent odor. Since the soil is not particularly nutritious, mangroves have adapted by modifying their roots. Prop root systems allow mangroves to take up gasses directly from the atmosphere and various other nutrients, like iron, from the otherwise inhospitable soil. They quite often store gasses directly inside the roots so that they can be processed even when the roots are submerged during high tide.

(Adapted from Hutchings and Saenger, 1987).

 

The open water area:
The bottom of the open area of the bay is primarily covered by seagrasses (Thalassia testudinum, Syringodium filiforme, Diplanthera wrightii, Ruppia maritime) macro-algae (Halimeda Punta, Avrainvillea nigricans, Acetabularia crenulata, Batophora oerstedi) and sand mounds, which are mainly created by marine worms and other bottom organisms.

Seagrass communities are very important ecosystems. They provide food, shelter, breeding and nursery grounds for a variety of marine organism, and play an active role in stabilizing the sediments as well.

Seagrasses are true flowering plants that have adapted to live in submerged marine environments. The anatomy of a seagrass is similar to that of any other grass. The horizontal rhizome anchors in the stem and seagrass blades that push up through several centimeters of sediment. The rhizomes anchor the plant in soft sediment and often intertwine to form dense mats. The growth of seagrass is dependant on the sediment depth. As the sediment depth increases, the seagrass bed elevation will increase to form a bank, the leaf density will increase and the leaf length will increase (Lott, 2001)

Within seagrass communities, a variety of benthic algae, phytoplankton and epiphytic algae growing on the seagrass blades can be found. Epiphytes are organisms living on the stems and leaves of seagrasses. Older leaves may be completely covered with epiphytic algae. A variety of animal species use the seagrasses as food source, like the Queen Conch (Strombus gigas), sea urchins, reef fishes, and sea turtles.

 

The coral dam:
The coral dam of Lac forms the border between the open ocean and Lac. Especially during high tide the influence of the open ocean is noticeable. Big waves enter the bay, which increases the current significantly; therefore this area is only accessible for species, which are well adapted to these specific circumstances (seaweeds and (coralline) algae).

Seaweeds are marine algae: saltwater-dwelling, simple organisms that fall into the rather outdated general category of "plants". Most of them are the green, brown or red. Unlike many marine animals, marine plants are limited to the shallow sunlight depths. Marine plants serve as food either directly for all the sea’s herbivorous animals or indirectly for most other marine animals. They are the food for the reef’s grazing animals including snails, crustaceans, sea urchins and fish.

In the area of Lac you will find seaweeds that are attached by holdfast, which just have an anchorage function. Another marine plant that attracts the attention immediately is the Sailor’s eyeball (Valonia ventricosa). This smooth shiny green ball (up to 6 cm) lives solitarily or in clumps on the reef flat.

Algae are commonly a part of the reef community. Living coral owes its brown, yellow or green color to algae. Algae and coral live together in a mutually beneficial relationship. The algae live inside the coral polyps and perform photosynthesis, producing food that is shared with the coral. In exchange, the coral provides the algae with protection and access to light, which is necessary for photosynthesis.